One of the methods for fabricating semiconductor elements is to form a semiconductor layer on a semiconductor substrate with an insulation layer already formed directly on its surface and then fabricate semiconductor elements in the semiconductor layer. In particular, the method in which a so-called SOI (Silicon On Insulator) substrate is used is drawing attention as a method for fabricating semiconductor elements. The SOI substrate includes a Si single crystal layer on an insulation layer. The Si single crystal layer is a thin film with a thickness of several micrometers, and the accuracy of the thickness of the Si single crystal thin film depends on the mechanical accuracy of the surface grinder and the polishing accuracy. Usually, the thickness of the Si single crystal thin film is within .+-.0.5 micrometers of the target absolute thickness.
However, in order to improve the performance of the semiconductor elements and mass-produce them with good yield, it is desirable to have the dispersion of the local thickness of the semiconductor layer such as a Si single crystal thin film within .+-.0.1 micrometers of the target absolute thickness. It is impossible to sufficiently improve the performance of the semiconductor elements with the conventional dispersion of .+-.0.5 micrometers.